Multilayered composite materials are generally well known. The individual layers of a composite material can typically be differentiated by the material type as well as the different structural properties of the materials (e.g., porosity). More recently, multilayered composite materials utilizing metallic and non-metallic foams have been developed to provide materials having good insulating properties as well as lightweight structures made from these materials. For example, German publication DE 39 05 871 A1 describes a highly porous inorganic material for use as thermal insulation in a three-dimensional composite material.
Such composite materials or structures made from such composite materials are typically characterized by high rigidity and low density or weight and may be used for any application requiring these characteristics such as, for example, structural parts in automotive and aerospace applications. Additionally, the above-mentioned composite materials also typically provide low thermal conductivity and good mechanical and acoustical damping characteristics, which enable their use as thermal and acoustical insulating materials as well as for the absorption of mechanical energy.
Processes for producing known types of composite materials and structures made from these materials are described in numerous publications. For example, German publication DE 44 26 627 C2 describes a process by which one or more metallic powders are mixed with one or more expanding agent powders. These metallic powder mixtures are typically compressed via axial hot pressing, hot-isostatic pressing, or rolling and, in a subsequent operation, are joined with previously surface-treated metal plates or outer cover layers through roll-bonding to form composite materials. The composite materials may then be formed or reshaped into semi-finished products or structures by, for example, pressing, deep drawing, bending, etc. The semi-finished products or structures are then heated to a temperature that lies in the solid/liquid range (e.g., somewhat near or above the melting point) of the metallic powder, but below the melting point of the metal outer layers. As the metallic powder is melted, the expanding agent powder, which is intermixed with the metallic powder, undergoes gas separation (i.e., outgasses). The gasses generated in this manner are essentially trapped bubbles that form expanding closed pores within the viscous core layer (i.e., the melted or melting metallic powder), which results in a corresponding increase in the volume of the core layer. The foamy (i.e., porous) core layer is then stabilized by a subsequent cooling operation.
European publication EP 1 000 690 A2 describes a modification of the process described in German publication DE 44 26 627 C2, in which the powder pressing is already designed in a closed porous manner. In particular, EP 1 000 690 A2 describes the production of a composite material based on a powder preform (i.e., a preformed or pressed core layer) that is initially produced to have open pores, which later become closed pores during a subsequent roller plating with the cover (e.g., outer metal) layers. The initial open porosity characteristic of the core layer serves to prevent potential gas separations of the expanding agent during its storage from causing geometric changes (e.g., expansion) of the preformed core layer, thereby preventing problems during the subsequent process of bonding the cover layers to the core layer. Furthermore, the initial open porosity characteristic may facilitate the break up of the oxide layers that may be formed during storage of the preformed core layer when the core layer is joined with the cover layers to form the composite material.
Another process for producing foamy or foamed composite materials described in German publication DE 41 24 591 C1, injects a metallic powder mixture into a metallic hollow section and performs a subsequent rolling operation to join the powder to the surfaces of the metallic hollow section. The reshaping of the semi-finished product and the subsequent foaming procedure are then performed in the same manner as described above in connection with German publication DE 44 26 627 C2.
European publication EP 0 997 215 A2 describes yet another process for producing a metallic composite material having solid metallic cover layers and a porous metallic core having a closed-pore characteristic. As described in EP 0 997 215 A2, the production of the compressed core layer and the fusion of the cover layers with the core layer are performed in one step. In particular, the powder mixture is introduced into the roll gap between the two cover layers and is compressed between the cover layers during the rolling process. In addition, this publication also suggests that the powder should be supplied in an inert gas atmosphere to prevent the formation of oxide layers that may adversely affect the fusion between the cover layers and the powder mixture that forms the core layer.
In another process described in German publication DE 197 53 658 A1, the operations associated with the production of the composite between core and cover layers, on the one hand, and the foaming, on the other hand, are combined. In particular, the core is introduced between the cover layers in the form of a powder preform and first joins with these layers as a result of the foaming procedure. As a result of the pressure supplied during the foaming of the core, the cover layers are subjected to reshaping in accordance with a form encasing them.
U.S. Pat. No. 5,972,521 A describes a process for producing a composite material blank in which air and moisture are removed from the powder through evacuation. Before the powder is compacted and joined with the cover layers, the evacuated air is replaced with a gas that is inert with respect to the core material and that is under increased pressure.
All of the above-described known processes imbed and compress (during compaction) a gas (e.g., air, an inert gas, etc.) in the core layer during its production. During the foaming process, the increased temperatures associated therewith may cause the imbedded gasses to form pores before the melting point of the metallic powder is reached. As a result, a substantial number of open, fissure-shaped, interconnected and irregularly-shaped pores may be formed instead of or in addition to more desirable closed, spherical pores, which provide better load transfer characteristics and, in general, a more stable composite material. While, for example, a process described in United States publication U.S. Pat. No. 5,564,064 A1 strives for open porosity through the expansion of enclosed gases below the melting temperature of the powder material, such open-pore composite materials are not well-suited for use in many structural applications.
Further, the open, fissure-shaped pores that are formed at temperatures below the melting point of the powder material as described above act as collecting areas for the gases created by the outgassing of the expanding agent. This compromises the homogeneous formation of preferably uniform, closed pores, which provide desirable homogeneous material properties such as, for example, high material stability. Because only the melting temperature of the core layer and not that of the cover layers is reached during the foaming process, the gasses produces by the expanding agent are prevented from escaping through the cover layers. This may lead to the formation of irregularly shaped, large-volume pores in the area immediately adjacent to the cover layers, which prevents an optimal connection between the core and the cover layers and may lead to localized delamination or separation of the core and cover layers.
The atmospheric humidity to which the powder material of the core layer is exposed during storage, transport and processing may cause an effect similar to that caused by the gases imbedded in the core layer during compaction of the core material. Due to the hydrophilic behavior of numerous powders, ambient humidity or moisture, which evaporates at relatively low temperatures (e.g., far below the melting temperature of the core material) when heated, is imbedded and, as described above, can lead to the formation of irregular, open pores with the associated performance disadvantages.
While the porosity resulting from the outgassing of the expanding agent powder in the viscous solid/liquid range of the core layer is held homogeneous to the greatest extent possible by ensuring that the expanding agent powder has a low grain size distribution and is evenly (homogenously) mixed with the metallic powder of the core material, undesirable open pores caused by gas inclusions and humidity as described above are largely uncontrollable.
The uncontrollable and undesirable open pore condition is typically responsible for the distribution or variation of material characteristic values within a production series. Such production process variability generally results in inadequate reproducibility of composite materials that are made of solid metallic cover layers and a closed porous metal core material as described above. Additionally, the presence of air and moisture also encourages the formation of oxides, which can compromise the metallic bond between the powder particles of the core layer as well the bonds between the core layer and the cover layers
Finally, European publication EP 0 927 590 A2 describes a composite material structure and fabrication process that is designed to prevent gas accumulations and the irregular porosity and related local delamination spots. In particular, the cover layers are perforated at (e.g., via punched holes) at regular intervals to enable the escape of gases generated during foaming of the core material. However, perforating the cover layers in this manner may structurally compromise the resulting composite material and/or may be undesirable for aesthetic/visual reasons in many applications.